Where to publish replications of machine learning papers?
OpenReview (https://openreview.net/) may be what you're looking for. It's both a platform for ML conferences to organize their reviewing process as well as an arXiv-like preprint repository.

One major machine learning conference (ICLR) has been holding "reproducibility challenges," in which participants can submit replication reports of existing papers, but that probably only applies to papers submitted to that conference.

There is a general preprints section, which might be your best bet.

Disclaimer: I used to be on the OpenReview development team